Currently, as a communication system which complies with a wireless communication standard employing a communication system of TDMA-TDD (Time Division Multiple Access-Time Division Duplex (hereinafter abbreviated as “time division multiplex”)), PHS (Personal Handyphone System) and i-Burst (registered trademark) system are mainly exemplified. These specifications are defined in ARIB STD-28 of Association of Radio Industries and Businesses (ARIB). Particularly, in American National Standards Institute (ANSI)/ATIS (Alliance for Telecommunications Industry Solutions), the i-Burst system is prepared under the name of HC-SDMA (High Capacity-Spatial Division Multiple Access) and being standardized by WTSC-2005-032.
Particularly, the i-Burst system uses an adaptive antenna array technique as the element technology. In this system, a base station 10 is provided with at least a plurality of antennas, as shown in FIG. 6. When each antenna of the base station 10 receives radio waves from a wireless communication terminal 1, the base station 10 analyzes the reception condition of the received radio waves and adaptively controls a radio wave reception level and a phase of each antenna according to the change of radio wave environment. As shown in FIG. 6, the base station 10 directs a point with maximal directivity to a direction where communication is to be performed (beam forming) so as to emphasize a desired wave, while it allocates an anti-phase wave in a direction of interference wave and forms a point where the directivity is declined (null point) so as to avoid interference. Therefore, disturbing waves from other systems or other terminals are removed and thus interference can be suppressed, which enables the favorable communication environment with a high data rate.
Moreover, this system also uses SDMA (Space Division Multiple Access (hereinafter abbreviated as “space multiplex”)) technique as the element technology. This expands the adaptive antenna array technique to a plurality of beams, and thereby a plurality of wireless communication terminals 1, 2 and 3, for example, can be accommodated in the same channel in a cell covered by the base station 10, as shown in FIG. 7. This technique makes it possible to communicate different data with other terminals at the same timing using the same frequency, as disclosed in Japanese Patent Application Laid-Open No. 2001-507889 and Japanese Patent Application Laid-Open No. 2002-58061, for example. In fact, PHS, HC-SDMA and the like use these techniques.
Next, the channel configuration proposed as HC-SDMA by WTSC-2005-032 of ANSI/ATIS will be explained with reference to a schematic view of FIG. 8. In HC-SDMA, Frequency Division Multiple Access (FDMA) divides a 5 MHz frequency band to eight frequency channels of f1 to f8, each of which has a frequency of 625 kHz.
Moreover, each of frequency channels f1 to f8 is provided with three time slots in the depth direction of the drawing by Time Division Multiplex (TDM). The time axis in the depth direction of the drawing, which is enlarged as a horizontal axis, is shown in the lower portion of the drawing. Each frequency channel is provided with three Uplink slots, each of which is of 545 μs, and three Downlink slots, each of which is of 1090 μs, so that Time Division Duplex (TDD) is achieved.
Furthermore, space multiplex enables a base station to communicate with a plurality of terminals at the same time on each of frequency channels f1 to f8. Hereinafter, the channel by this space multiplex (Spatial Channel) will be abbreviated as simply “SC”, and when the base station can communicate with three terminals at the same time on each frequency channel, for example, the frequency channel is represented as “3SC”.
Therefore, in the whole of 5 MHz frequency band, as shown in FIG. 9, each of eight frequency channels has three time slots and further each sub channel is 3SC, and thus the theoretical channel is of 8[Frequency Channels]×3[Time Slots]×3[Spatial Channels]=72[streams]. In fact, since the first time slot in a specific frequency channel is used as a control channel, the channels on which a base station actually connects terminals and which can be used by a user for communication are 69 [streams].
Therefore, the data rate of each stream can reach 354 kbps/115 kbps (Downlink/Uplink) when the radio wave condition is favorable.